What I would do is to create a path that is a circle and use that with a CAShapeLayer and animate the strokeEnd similar to what I did in this answer.
It would look something like this (but I didn't run this code so there may be typos and other mistakes):
UIBezierPath *circle = [UIBezierPath bezierPathWithArcCenter:center
radius:radius
startAngle:0
endAngle:2.0*M_PI
clockwise:YES];
CAShapeLayer *circleLayer = [CAShapeLayer layer];
circleLayer.bounds = CGRectMake(0, 0, 2.0*radius, 2.0*radius);
circleLayer.path = circle.CGPath;
circleLayer.strokeColor = [UIColor orangeColor].CGColor;
circleLayer.lineWidth = 3.0; // your line width
CABasicAnimation *drawAnimation = [CABasicAnimation animationWithKeyPath:@"strokeEnd"];
drawAnimation.duration = 10.0; // your duration
// Animate from no part of the stroke being drawn to the entire stroke being drawn
drawAnimation.fromValue = @0;
drawAnimation.toValue = @1;
Just note that both the path and the shape layer has a position so the circle path should be defined relative to the origin of the shape layers frame. It might be clearer to define the shape layer first and then create an oval inside of its bounds:
CAShapeLayer *circleLayer = [CAShapeLayer layer];
circleLayer.bounds = CGRectMake(0, 0, 2.0*radius, 2.0*radius);
circleLayer.position = center; // Set center of the circle
// Create a circle inside of the shape layers bounds
UIBezierPath *circle = [UIBezierPath bezierPathWithOvalInRect:circleLayer.bounds];
circleLayer.path = circle.CGPath;
// Same appearance configuration as before
circleLayer.strokeColor = [UIColor orangeColor].CGColor;
circleLayer.lineWidth = 3.0; // your line width